A developing device provided in an electrophotographic or electrostatic recording type image forming apparatus generally uses a two-component developer mainly containing toner particles and carrier particles. In particular, in color image forming apparatuses for forming a full-color image or a multi-color image, most developing devices use the two-component developer. The toner density (that is, the ratio of the weight of the toner particles to the total weight of the carrier particles and toner particles) of the two-component developer is a very important factor for image quality stabilization.
Upon development, the toner particles of the two-component developer are consumed, and the toner density changes. For this reason, a technique (PTL1) has been disclosed which detects the toner density of a two-component developer in a developing device and controls toner supply to the developing device in accordance with the detected toner density, thereby controlling the two-component developer to maintain predetermined toner density.
However, the above-described method cannot always output an image at a desired density. One major reason for this is a variation in the toner charge amount. The toner charge amount is one of the important factors for image quality stabilization. Electrophotography or electrostatic recording forms an image using the electrostatic force. For this reason, a variation in the toner charge amount leads to a variation in the image density.
Known causes of the variation in toner charge amount are temperature and humidity in the environment where the image forming apparatus is installed and aging degradation of the carrier caused by long-term use. Another main cause is a change in toner consumption on images.
FIG. 10 is a graph showing an example of a change in the toner charge amount caused by agitation. Leaving toner to stand for a long time causes frictional electrification as the toner is agitated and rubs against the carrier in the developing device. An example of the change in the toner charge amount corresponding to toner consumption when 20 document pages are printed will be described with reference to FIGS. 11A to 11C.
FIG. 11A is a graph showing the toner consumption of each printed sheet in the example to be described based on FIGS. 11A to 11C. The toner consumption of each sheet is 2 T (mg) when printing the first to 10th pages and T (mg) when printing the 11th to 20th pages. FIG. 11B is a graph showing the toner supply amount for each sheet. The toner is supplied in the same amount as the consumed amount in development. FIG. 11C is a graph showing the toner charge amount at the start of printing of each sheet under the circumstances illustrated in FIGS. 11A and 11B.
Before submitting a print job, the toner is sufficiently agitated, and the toner charge amount is 30 Q (μC/g). When the print job is executed, new toner that is not sufficiently frictionally electrified is supplied to the developing device. The toner charge amount gradually decreases because frictional electrification by agitation in the developing device cannot keep up. The toner charge amount thus converges to almost 23 Q (μC/g). From the 10th page where the toner consumption and supplied toner amount decrease, the balance between the supplied toner and the toner remaining in the developing device changes, and the toner charge amount gradually increases and converges to almost 27 Q (μC/g).
As described above, even when the conditions of the toner density and output environment are controlled to predetermined levels, the toner charge amount may change between output images. Since the image density also changes with variation in the toner charge amount, it may be impossible to output a document at a desired density. To solve this, a method is used which detects the density of a developed image and supplies toner if the density is lower than a desired value. There is also a method of correcting the grayscale of an image signal instead of controlling toner supply (PTL2).